1. Field of the Invention
This invention relates to an improved crane structure for a bicycle pedal, more particularly to a crank structure having a cylindrical sleeve in an inner thread hole of a crank and two threads respectively disposed at a front section and a rear section on the outer periphery of the cylindrical sleeve and substantially in opposite rotary directions with each other to respectively limit the screw connection of the inner thread of the crank and the nut in order to attain the effects of securely mounting and easily detaching the crank structure.
2. Description of the Related Art
In general, a bicycle A provides a crank C for coupling a pedal B as shown in FIG. 1A. In the figure, a hole is disposed each at a front section and a rear section of the crank C, and the hole at the front section is coupled with an axial rod A1 of the bicycle body, and the inner thread hole C1 at the rear section is coupled with a bolt B1 protruding from the pedal B by a screw in order to provide a pedaling movement by the rider's legs. However, the entire body of the present bicycle emphasizes light weight. Therefore, traditional steel material is no longer adopted, but a light-weight aluminum alloy material is used instead, and such material also applies to the crank C naturally. However, the crank made of aluminium alloy material is unable to withstand the strong force exerted on the inner thread hole C1 at the rear section of the crank C, and thus manufacturers usually install a wear-proof cylindrial sleeve D onto the inner thread hole C1 in the crank C. An outer thread D1 formed on the outer periphery of the cylinidrical sleeve D has the same rotary direction with the inner thread hole C1 in the crank C; and an inner thread D2 formed on the inner wall has the same rotray direction with the bolt B1 that protrudes from the pedal B, such that after the outer thread D1 on the surface of the cylindrical sleeve D is secured into the inner screw hole C1 of the crank C as a whole, the inner thread D2 on the inner wall of the outer thread D1 is coupled with a bolt B1 protruding from the pedal B (as shown in FIG. 1B) to strengthen the stability of connecting the crank C with the pedal B with the assembling of the cylindrical sleeve D.
Although such method can attain the purpose of strengthening the resistance to shocks and vibrations, it also causes the cylindrical sleeve D to loosen and fall off due to the shocks and vibrations produced when the bicycle is running on an uneven road surface. Therefore an improved connection method as shown in FIG. 2 was suggested. After the cylindrical sleeve E is coupled, the end section E1 is spread out to attach onto the outside of the inner thread hole C1 of the crank C to constitute a positioning after the cylindrical sleeve is installed, but such method may damage the inner thread E2 at the end section E1 of the cylindrical sleeve E, and cause the bolt B1 protruding from the pedal B unable to be fully screwed for the connection.